1. Field of the Invention
This invention relates to a direct positive silver halide photographic emulsion.
2. Description of the Prior Art
Direct positive silver halide photographic emulsions are described in, e.g., British Pat. No. 723,019, U.S. Pat. Nos. 3,501,305 and 3,632,340, and Japanese Patent Publication No. 1326/73. Such photographic emulsions comprise an emulsion obtained by adding a desensitizing dye which acts as an electron trap to an emulsion fogged with a reducing agent and a gold compound, or an emulsion in which the surface of silver halide grains having internal nuclei that accelerate the formation of silver by photolysis is fogged with a reducing agent and a gold compound.
When a direct positive silver halide photographic light-sensitive material having, on a support, a layer of a photographic emulsion prepared in this way is exposed and then developed, the resulting silver image has high contrast and it is very difficult to obtain an image of low contrast. Moreover, in an emulsion containing an electron trapping agent inside of the silver halide grains, reversal occurs initially due to the breakdown of fog nuclei, and then negative sensitivity occurs, thereby deteriorating the linearity of the H-D curve (the characteristic curve). The fog nuclei formed in the above way disappear with time and the fog density gradually decreases, so the emulsions can not be stored for a prolonged time.
Japanese Pat. Publication No. 4935/68 discloses usual emulsions in which iridium salt is used and the disclosure relates to a method of chemical sensitization of the surface of silver halide grains by adding an iridium salt during the emulsification or the ripening of the silver halide grains. However, an object of this emulsion is to prevent, when the emulsion is subjected to flash exposure, the usual formation of lower contrast as compared with the case of conventional exposure.
Moreover, Japanese Patent Publication No. 1326/73 discloses a direct positive silver halide photographic emulsion in which an iridium salt is used. This emulsion comprises an internal latent image type emulsion, and the surface of silver halide grains is subjected to sulfur sensitization.